The present invention generally relates to a laser output control device for use in an optical information recording and reproducing apparatus that records and reproduces information onto and from an optical information recording medium, and it more particularly relates to a device for controlling output power of a semiconductor laser.
Semiconductor lasers provide light outputs that vary with changes in temperature and in luminous energy. On the other hand optical information recording and reproducing apparatus provide automatic power control (hereinafter referred to as APC) of laser light utilized therein, because they require constant laser light output for recording and reproducing information in a uniform condition. Examples of the prior techniques relating to such automatic control of laser light output are disclosed in Japanese Patent Laid-open Publication Nos. SHO 52-23902 and 51-29821, Japanese Patent Publication Nos. HEI Nos. 2-47015 and 5-6260, and Japanese Patent Laid-open Publication No. HEI 4-330646.
For example, in an optical information recording and reproducing apparatus as shown in FIG. 7, the peripheral light component 5 of forward radiated light output from a semiconductor laser chip 1 is eliminated via a collimating lens 3, so that only the non-peripheral light component 2, i.e., the light component in and around the center of the forward radiated light, enters the collimating lens 3 to be converted into a collimated light beam, which then enters a beam splitter 4. The non-peripheral light component 2 is divided into transmitted light 6 and reflected light 7 by the beam splitter 4. The transmitted light 6 is converged by a converging lens 8 and directed onto an information recording medium 9 as a very minute laser light spot 10 to be utilized for effecting information recording or reproducing as needed. The reflected light 7 from the beam splitter 4 enters a light receiving element 15.
Reflected light from the information recording medium 9 goes back the same path that the transmitted light 6 took to reach the recording medium 9. The reflected light is converted via the converging lens 8 into a collimated beam, which is then directed to the beam splitter 4. Light transmitted through the beam splitter 4 is then directed onto the semiconductor chip 1 as return light, while light 11 reflected by the beam splitter 4 is received by a light receiving element 13 via light receiving lenses 12. Information signal, focus controlling signals and tracking controlling signals are derived from the light receiving element 13. The APC executes output control to maintain the laser light output at a constant value by means of a loop circuit configured such that a part of the light output from the semiconductor laser is received by the light receiving element 14 or 15, the light output from the element 14 or 15 is amplified by an output control circuit and fed back to the semiconductor laser chip.
To this end, the light receiving element 14 is designed to receive rearward radiated light 16 from the semiconductor laser chip 1 so as to provide the light output 17, and the light receiving element 15 is designed to receive the reflected light 7 from the beam splitter 4 to provide the light output 18.
In the illustrated embodiment, the rearward radiated light 16 output from the semiconductor laser chip 1 is derived via the light receiving element 14 that is mounted within a semiconductor laser housing (not shown). However, this prior arrangement presents a problem, because the forward and rearward radiated light outputs from the semiconductor laser chip 1 are not in complete correlation with each other, can not maintain the laser light output at a constant value and hence can not provide satisfactory APC operation.
Further, because the light receiving element 15 receives the reflected light 7 from the beam splitter 4, the element 15 greatly depends on the characteristics of the beam splitter 4. Thus, if, for example, a polarizing beam splitter is employed as the beam splitter 4 to reduce the transmitted light loss in the splitter and improve the optical efficiency of the optical system (usually, the beam splitter is combined with an 1/4 wave plate so as to constitute a light isolator), the reflected light 7 will not exist, so that no output can not be obtained from the light receiving element 15.
Furthermore, the light receiving element 15 becomes large in size because it is designed to receive the reflected light 7 directly from the beam splitter 4, and thus the interelectrode capacity of the element 15 is increased, with the result that the response speed is objectionably decreased. This problem may be overcome by converging the light incident on the light receiving element 15 so as to reduce the size of element 15. To do this, it is necessary to provide a new light converging means, but provision of the new converging means is not compatible with the current demand for reducing the size and cost of the apparatus.